This application is a competitive renewal of DA10309, which examines the role of Homer proteins in the cell biology of activity-dependent plasticity and drug addiction. With the support of the prior award, we have demonstrated that Homer binds to proline rich sequences in group 1 metabotropic glutamate receptors and regulates aspects of mGluR function, mGluR signaling appears to play an essential role in addiction since mice that do not express mGluR5 do not show behavioral activation to cocaine, and do not self administer. Homer has provided a powerful new vantage to understand aspects of mGluR signaling that have been reported in physiological studies but are not currently understood at a molecular level. In Aim 1, we propose to study molecular mechanisms that control the binding of Homer to mGluR. Ongoing studies indicate that Homer binding is complexly regulated by phosphorylation of mGluR at the site of Homer binding, and involves the coordinated actions of protyl isomerases and calcineurin. This phosphorylation appears to be an important point of crosstalk with dopamine receptors and growth factor receptors, and these mechanisms will be explored in Aim 2. Aim 3 will examine Homer-dependent coupling of mGluR to a novel endocytosis pathway that traffics membrane ionotropic glutamate receptors and growth factor receptors. This pathway is hypothesized to underlie mGluR-dependent LTD. Aim 4 will examine coupling of mGluR and Homer to factors involved in protein translation and that may underlie local protein synthesis at active synapses, Aim 5 will examine coupling of mGluR1 to cation channels (termed TRPC channels) that regulate the fill state of Ca2+ stores, and that mediate a mGluR-dependent current that is essential for long-term depression in cerebellar Purkinje neurons. Physiological studies will examine the role of each of these pathways (Aims 1-5) in mGluR-dependent synaptic plasticity. These studies build upon a knowledge base of protein structure and protein-interaction chemistry to define the mechanisms and physiological functions of mGluR signaling. Studies of mGluR-TRPC channel function may also provide insight into the elusive role of intracellular Ca stores in neuronal synaptic physiology. [unreadable] [unreadable]